autotest/virt-test — pull request #1878 by PandaWei (2014-11-12 02:52:10+00:00)
Title: libvirt_xml: add try/except for remove_by_xpath() failure

del_devices() and remove_all_graphics() maybe suffer from exceptions.
such as, AttributeError and TypeError.
If no element in specified xml file and deleting operation fails,
then use the except block to ignore the error.